The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge.
U.S. Pat. No. 4,739,786 and its related publications derived from WO86/02725 describe a monitoring assembly which has a plurality of sensors arranged in spaced relationship along a body which may be suspended from a coded multicore cable in a reservoir to monitor liquid levels therein. Each sensor is connected to a respective coded core of the multicore cable such that any sensor may be selectively coupled to a remote control apparatus. The output may be used to operate level alarms, pumps or the like.
An inherent difficulty which occurs with liquid level sensors in practical applications such as waste water management lies in the inhomogeneous nature of the medium in which they operate. Entrained fats and oils, and solids such as precipitated salts and grit, provide that the sensors may be encapsulated or otherwise rendered inoperable. Multiple sensor elements may be adapted to provide some diagnostic functions whereby occlusion of a sensor relative to one or more other sensors may be logically interpreted as indicative of a need for maintenance such as cleaning.
The monitoring apparatus provides reliable sensing at each sensor station along the body, but is not a continuous analogue level output device by virtue of the sensors being spaced along the body. Accordingly, any liquid level change between each pair of adjacent sensors cannot be detected or monitored. The liquid level above the topmost sensor and below the bottommost sensor is also not detected or monitored by the device, which also means that a power-down state of the sensors per se may go undetected. There remains a need for sensors to be more indicative of the need for maintenance and for apparatus to provide continuous analogue or digital output.
There have been attempts to use a pressure sensor, bubbler or other analogue device in a pit with a probe as described above. This approach has been unsuccessful due to the difficulty in reliably installing a pressure sensor low enough in a sewerage well to be effective. Although the probe operates well at a low liquid cut off level, the pressure transducers are unreliable at this low liquid level. Turbulence is extreme at the last stages of pump out which effects liquid density so that presently available pressure sensors cannot be used at this low level. The pressure sensor obviously cannot be used at the lower (pump cut-off) level since the signal disappears into noise as the head pressure approaches zero. One approach to overcoming this problem that has been taken in the past is to mount the sensor higher than optimum. However, complete pump out of the well cannot be achieved accurately due to the inability to accurately calibrate the sensor in the highly variable environment. A higher cut off level does not evacuate the pit of liquid effectively, thereby resulting in excessive fat and debris build up due to it floating and concentrating in the top layer of liquid.
Arbitration techniques including a redundant low-level cutout sensor installed independently are known, and can result in pumps not being turned off at the appropriate time resulting in pump damage.
When installing two level sensor devices in the same pit, it is desirable to have a fixed physical unchangeable relationship between the two devices so that arbitration can be carried out in order to detect incorrect level measurement from either of the devices. To do this, the analogue sensor has to be a set distance below the bottom sensor on the probe and also, the controller needs also to know the distance between all other sensors accurately so it can calculate and compare continuously level height from each device. There can be many models and lengths of sensing probes that can be connected to a controller. In addition, an installation may have hundreds of sites so variables including sensing distances of analogue probe sensors, different models and lengths of probes, and changeable set points for pump starts/stops make the option to install independent systems unviable.
It is very difficult to physically to reliably install a separate pressure transducer at a consistent fixed distance say 60 mm lower than the bottom sensor of the probe. Since the pump cut-out point may be over 10 meters from the pit opening, workplace health and safety considerations will typically prohibit people climbing into a pit, especially at that depth, to ensure both the probe and the pressure transducer are at the right levels. In any case, both the probe and sensor need to be removed for cleaning on a regular basis so there will be a difference in levels due to reinstallation and also stretching of the sensor's cable or movement due to turbulence affects the position of the pressure transducer. Also, when the pump station is reaching the low pump out level, turbulence and water action can be extreme, so a separate pressure sensor unit will be violently washed around and damaged by the turbulence. Currently, pressure transducers are mounted much higher than the optimum level to try to overcome problems and damage due to turbulence but, even so, many transducers are still damaged from turbulence and extreme water flows found in sewerage pumping stations.
Fixing a separate pressure sensor to or near the probe, seriously affects the operation and reliability of the probe as excessive build up gets trapped between the sensor cable and probe.
The duty point on the probe is a problem because this is where build up occurs which may eventually cause the probe to fail. This is the zone where a pressure transducer may be favored, with sufficient depth to provide a pressure head and a useful signal to noise ratio. However, the previously described calibration problem remains. This duty area is very important when calculating volume pumped as a variation of the start point due to fat build-up compounds errors in calculating volume pumped. That is, if the pump is activated before the liquid level falls to that of the sensor then the controller logic will determine that the volume increased dramatically at the last stage during fill up mode and will use this inflated figure to calculate pump down volume.
Because high level alarms are rare, probe sensors and connections are not confirmed regularly. This area is critical from an environmental protection agency (EPA) perspective. Currently, once level is beyond the top of the probe, no more monitoring is available.
Currently, deploying dual back up systems of any kind is problematic. Dual back up systems are very difficult to commission and are subject to continual operational changes, which result in unreliability, false trips and system failure, mainly due to arbitration to which is the correct reading. During arbitration in systems including two independent level sensors, it is difficult to determine which sensor reading is correct in the event that there is a discrepancy between sensor readings. Set points for pump starts, stops, alarms are changed regularly by operators and calibration of the sensors is required when the set points are changed. Currently, skilled installers are required to physically install two level sensing devices accurately. Technical personnel then have to set up complex control and monitoring systems with arbitration and then hope that nothing moves due to cleaning, maintenance procedures or configuration changes, otherwise the commissioning and set up has to all be repeated.
It is an object of the present invention to address one or more of the foregoing problems.